Controlled linear thermal expansion alloys, some forms of which have been identified as Incoloy 903(IN903) alloy, CTX I alloy and CTX II alloy have potential use in advanced gas turbine engines. Such alloys possess unique thermal expansion characteristics which can improve specific fuel consumption by maintaining closer operating tolerances. For example, articles which can be made from such alloys include seals, shroud supports and hangers, as well as turbine casings. Such an alloy is characterized as having an inflection, or significant change, in its mean coefficient of linear thermal expansion in its curie temperature range.
One characteristic of such alloys is that Cr has not been added as an alloying element, although it may be includes as an impurity up to about 1 weight percent. Therefore, the environmental resistance of such alloys is relatively poor. As a result, a protective coating is required for application of such alloys in gas turbine engines.
The mechanical properties of such alloys have been found to be sensitive to the environment, particularly in the recrystallized condition. Additionally, stress accelerated grain boundary oxidation is believed to contribute to the sensitivity of the mechanical properties to the environment and microstructure. Therefore, a suitable coating is required to retain the mechanical properties of such alloys, particularly of the recrystallized material.
It is a principal object of the present invention to provide an article of a controlled linear thermal expansion alloy, the surface of which has been provided with environmental resistance.
Another object is to provide a method for making such an article without recrystallizing its microstructure.
These and other objects and advantages will be more clearly understood from the following detailed description and examples which are intended to be typical of rather than in any way limiting on the scope of the present invention.
Briefly, the present invention, in one form, provides a metallic article comprising an Fe--Ni base alloy substrate of the controlled linear thermal expansion type, characterized by the substantial absence of Cr and having a mean coefficient of linear thermal expansion of less than about 4.7 inches .times.10.sup.-6 per inch per .degree.F. (8.5 mm .times.10.sup.-6 per mm per .degree.C. at the inflection temperature in the range of about 780.degree.-880.degree. F. (416.degree.-471.degree. C.) In one form, such alloy consists essentially of, by weight, 30-40% Ni, 10-20% Co, 1-5% of the sum of Cb and Ta, 0.5-3% Ti, 0.2-3% Al, up to about 3% each of Hf and Zr, up to about 0.5% B, with the balance essentially Fe and incidental impurities, the substrate having diffused therein a material selected from Cr and its alloys. In a preferred form, such alloy substrate consists essentially of, by weight 35-40% Ni, 13-17% Co, 2-4% of the sum of Cb and Ta, 1-2% Ti, 0.3-1.2% Al, up to 3% Hf, with the balance Fe and incidental impurities. It is preferred that such impurities be maintained in a range up to a maximum of 0.012% B, 0.05% Cu, 0.06% C, 1% Mn, 0.35% Si, 0.015% S, 0.015% P and 1% Cr.
According to the method associated with the present invention, such coated metallic article is provided by diffusion chromiding the article surface in a container with a non-oxidizing, preferably reducing atmosphere such as H.sub.2, and a powdered mixture comprising, by weight, 10-50% Cr powder, 0.1-4% of a conventional halide salt activator, particularly a chloride type such as NH.sub.4 Cl or CrCl.sub.3, with the balance of the mixture being an inert powder filler such as Al.sub.2 O.sub.3. Preferably such mixture consists essentially of, by weight, 15-25% Cr, 1.5-2.5% of a chloride salt activator, with the balance Al.sub.2 O.sub.3. Such a method is conducted at a temperature below that which will recrystallize such an alloy, generally less than 1700.degree. F. and preferably in the range of about 1450.degree.-1650.degree. F.